You should start auto trace from the first frame. You might have more success if you pre-composed your keyed layer moving all attributes to the new comp and then tried to auto trace the nested comp using the alpha channel,
I'm not sure why you are auto tracing your layer when you already have a good mask from Keylight. It might help if you explained what you are trying to accomplish.
The problem is still the same, precomposed layer and traced from 1st frame, but it still crops the mask
I want to key everything out of my body, so keylight won`t be searching for green color there
Before this video I`ve done the same and it worked, but now it is not.
How to: Perfect Chroma Keys from Bad Green Screens. - YouTube
I tried idea from this video
Unless your footage is terrible this is not necessary and there are a lot better ways to garbage matte your video that and auto trace is about the worst tool I can think of to create a hold out matte. That tutorial demonstrates one of the least effective and efficient ways I have ever seen demonstrated to use Keylight. You need to do a better job of vetting your trainers. Google is becoming the worst way to find good training. For every 10 YouTube videos I see referenced in this forum that people are having problems with 8 or 9 of them are demonstrating poorly thought out and inefficient techniques. That's the problem when the masses get their hands on professional software with about 100,000 switches under the hood.
Show us the footage or at least a frame of the original footage and we can make some suggestions that will point you in a more effective direction. Check out the manufacturer or sites like Video CoPilot or Lynda.com to find some really good training.